With the advancement of electronic devices in size reduction and high-density packaging, there has been a strong demand in recent years for multi-layering of printed circuit boards not only in the field of industrial electronics but also in the field of consumer electronics.
For such printed circuit boards, it is indispensable to newly develop methods of joining inner via-holes between multilayered circuit patterns in addition to highly reliable structures. Therefore, certain methods have been proposed for manufacturing printed circuit boards of new structures and high-density packaging in which inner via-holes are joined with electrically conductive paste.
One of such conventional methods of manufacturing two-sided printed circuit boards will now be described hereinafter. FIG. 13A through FIG. 13F are cross-sectional views of a printed circuit board showing the production steps in the conventional manufacturing method, FIG. 14 a perspective view showing a plate framework of the prior art provided with a mask having an opening, FIG. 15 a longitudinal sectional view of the same plate framework provided with the mask having the opening, FIG. 16A through FIG. 16G cross-sectional views of a prepreg sheet showing the production steps for filling it with paste by the squeegeeing method, and FIG. 17 a cross-sectional view of a part of a printed circuit board which is being filled with paste.
In FIG. 13A through FIG. 13F, prepreg sheet 21 has a size of 300 mm by 500 mm and a thickness of approximately 150 μm, for example. Prepreg sheet 21 used here may be a board material of composite prepared by having an unwoven fabric formed of all-aromatic polyamide fibers impregnated with a thermosetting epoxy resin. Mask films 22a and 22b have a Si-based mold release layer formed into a thickness of 0.01 μm or less on each of their surfaces to be bonded to prepreg sheet 21. Each of mask films 22a and 22b has a total thickness of about 16 μm, and that a plastic film of 300 mm in width made of such material as polyethylene terephthalate may be used.
A method proposed here for adhesion between prepreg sheet 21 and mask films 22a and 22b uses a laminating apparatus which melts the resin content of prepreg sheet 21 and bonds it continuously to mask films 22a and 22b. Through holes 23 are filled with conductive paste 24 for making electrical connections with metal foils 25a and 25b of 35 μm in thickness made of copper, for instance, placed on both surfaces of prepreg sheet 21.
In the process of manufacturing the printed board, these through holes 23 are formed in predetermined locations of mask films 22a and 22b bonded to the both surfaces of prepreg sheet 21 by using laser beam machining or the like method, as shown in FIG. 13B.
Through holes 23 are then filled with conductive paste 24 as shown in FIG. 13C. Filling of conductive paste 24 may be made with such a method that prepreg sheet 21 having through holes 23 is placed on stage 6 (shown in FIG. 16A to FIG. 16G, as will be described later) of an ordinary printing machine (not shown in FIG. 13), and conductive paste 24 is filled directly over mask film 22a by reciprocating alternately two squeegees made of such a material as polyurethane rubber. In this process, mask films 22a and 22b on prepreg sheet 21 act individually as print masks, and they also serve as means for preventing the surfaces of prepreg sheet 21 from getting contaminated.
Description is provided further of the method of filling conductive paste 24 by referring to FIG. 14, FIG. 15, and FIG. 16A through FIG. 16G.
The squeegeeing method is used for filling of conductive paste 24. Since prepreg sheet 21 has exclusive mask films 22a and 22b, plate framework 1 of plate 10 for printing is provided with mask 2 made of an approx. 3 mm thick stainless steel having opening 4 measuring 250 mm by 450 mm, which is larger than an effective paste-filling area of prepreg sheet 21, as shown in FIG. 14 and FIG. 15.
To begin with, in preparation of filling conductive paste 24, prepreg sheet 21 formed with through holes 23 is placed on stage 6 of a printing machine (not shown), and mask 2 is set on top of them, as shown in FIG. 16A.
Next, between two squeegees, i.e., moving-forth squeegee 8a and moving-back squeegees 8b disposed above mask 2 in a manner to move forward and backward freely and to pressurize vertically, moving-forth squeegee 8a is lowered to a predetermined position on mask 2 and it is moved forward while pushing a drop of conductive paste 24 to roll about with a pressure.
Then, moving-forth squeegee 8a is advanced to pass over edge 5b of the opening of mask 2, and to reach on top of prepreg sheet 21, as shown in FIG. 16B. Both moving-forth squeegee 8a and moving-back squeegee 8b have the function of moving up and down freely with respect to position of prepreg sheet 21 while maintaining the pressure.
Subsequently, after moving-forth squeegee 8a sweeps over prepreg sheet 21 and the other slanting area and stops at another predetermined position on mask 2, it is raised and let conductive paste 24 drop naturally, as shown in FIG. 16C.
Next, only moving-back squeegee 8b is lowered to a predetermined position on mask 2 as shown in FIG. 16D. Afterwards, moving-back squeegee 8b is moved to sweep over mask 2 and prepreg sheet 21 in the like manner as moving-forth squeegee 8a, as shown in FIG. 16E to FIG. 16G, and this completes the filling of through holes 23 with conductive paste 24.
Mask films 22a and 22b are then removed from both surfaces of prepreg sheet 21, as shown in FIG. 13D. Next, metal foils 25a and 25b made of such material as copper are placed on both surfaces of prepreg sheet 21, as shown in FIG. 13E.
Prepreg sheet 21 in the above state is compressed to thickness “t2”(approx. 100 μm) as shown in FIG. 13F by subjecting it to a thermal compression. This process also bonds metal foils 25a and 25b with prepreg sheet 21. Consequently, metal foils 25a and 25b on the both surfaces are connected electrically via conductive paste 24 filled in through holes 23 provided in the predetermined locations.
Metal foils 25a and 25b are then etched selectively to form a prescribed circuit pattern (not shown in FIG. 13F), to obtain a two-sided printed board.
However, the above paste-filling technique of the prior art has a problem peculiar to it as shown in FIG. 17. That is, if conductive paste 24 has comparatively high viscosity, moving-back squeegee 8b presses conductive paste 24 against the surface of prepreg sheet 21 when it is lowered along edge 5b of the opening of mask 2 at the start of printing. This pushes out a resin content of the conductive paste 24 around the edge of moving-back squeegee 8b, and causes conductive paste 24 of high viscosity to adhere onto the entire area of the edge surface of moving-back squeegee 8b. 
When any of the squeegees bearing conductive paste 24, especially moving-back squeegee 8b, first passes over through holes 23 formed in prepreg sheet 21, moving-back squeegee 8b tends to leave hard paste on top of these through holes 23 across the entire filling width of the paste in its moving direction. It thus gives rise to a problem that a part of conductive paste 24 transfers to mask film 22a when mask films 22a and 22b are removed, and adversely influences quality of the product.
Prior art documents hitherto known as being relevant to the present invention include Japanese Unexamined Patent Publications, Nos. H06-268345, H07-106760 and 2001-213064, for instance.